(a) Field of the Invention
The present invention relates generally to fiber lasers and, more particularly to a fiber laser having asymmetric output ports.
(b) Description of Related Art
Fiber lasers are used in a variety of applications. As with all lasers, a fiber laser generates coherent light wherein the amplitude, polarization, frequency or wavelength, and phase of the output laser light can be controlled. In general, fiber lasers include an optical pump source, two reflectors comprising the optical cavity of a resonator, and an active region within the cavity. Unlike other lasers, the cavity and active region of a fiber laser are formed in an optical fiber. The fiber generally includes a doped glass core that acts as the laser's active region. In operation, the pump is coupled, via one end of the resonator, to the doped-glass core active region. The ions in the doped core are excited by the pump to generate light that is reflected between the reflectors. At least one of the reflectors of the resonator is partially reflective, thereby allowing a portion of the laser light to escape the cavity as the laser output.
Erbium is commonly used as a dopant for fiber lasers. The doped optical fiber is pumped with an optical source having a wavelength .lambda..sub.p. As the doped optical fiber is pumped above a threshold level it lases, emitting coherent optical energy at a wavelength .lambda..sub.s.
Fiber lasers have many uses in the communications industry including telecommunications or other systems using coherent detection methods. Fiber lasers may also be used as local oscillators in commercial radar applications. Recently, fiber lasers have been proposed for use in automobile collision-avoidance systems.
Two configurations of fiber lasers known in the art are the distributed Bragg-reflector (DBR) configuration and the distributed feedback (DFB) configuration. The DBR laser uses a DBR element and at least one other reflector, which is typically a DBR, to provide the necessary optical feedback for the narrowband lasing process. The DBR laser requires spectral alignment of the distinct DBR elements, which is difficult to achieve in a manufacturing situation. Alignment of the DBR elements is time consuming and expensive. A further disadvantage of the DBR laser is that it generally requires a long cavity length (relative to a DFB laser) to separate internal reflections that form in the cavity of the laser. The relatively long cavity length is also required to obtain proper spacing of the allowed lasing frequencies (longitudinal modes) with respect to the spectral bandwidth of net gain of the laser related to the bandwidth of the DBR elements and ensure one lasing frequency. Long cavity lengths in high erbium concentration doped fibers can lead to increased noise bursts during laser operation. These noise bursts are due to the groups of erbium ions that collect in a nonuniform manner within the cavity.
In DBR fiber lasers, it is known to adjust the reflector elements so that most of the output power flows from one end of the laser cavity. Typically only one of two output ports provides the necessary power output from the laser, which lowers laser cavity loss and laser threshold power for oscillation.
The distributed feedback (DFB) laser configuration uses a short cavity (relative to a DBR) because the grating reflector spans the length of the cavity, thereby distributing the optical feedback. The grating reflector used is the same length as an individual reflector element in an analogous DBR configuration with identical frequency discrimination. Due to the shorter cavity length there are fewer erbium ion clusters within the cavity as compared to the longer cavity of a DBR laser. Therefore, there is a lower probability of noise bursts during laser operation of a DFB fiber laser. The longitudinal mode spacing is approximately the same as the bandwidth of the grating reflectivity, which leads to ease of single frequency laser operation. Another advantage that the DFB laser has over the DBR laser is the elimination of the need to spectrally align distinct reflectors because only a single DBR reflector is used. Therefore, the DFB laser is easier to manufacture than the DBR laser. Asseh et al. discloses a DFB fiber laser in 10 cm Yb.sup.3- DFB fibre laser with permanent phase shifted grating, Electronics Letters, 8th Jun. 1995, Vol. 31, No. 12, p.969-70.
DFB fiber lasers are symmetric in nature, which precludes an asymmetric power flow design. That is, it is currently available technology does not allow on to design a DFB fiber laser that is symmetric yet does not output the same power from both ends of the laser.
It can be readily appreciated that the DFB laser configuration provides significant advantages over the DBR laser configuration. However, known DFB fiber laser configurations cannot be designed for asymmetric power flow, which is desirable in many control and communication situations.